Device for a method of growing monocrystals

ABSTRACT

A DEVICE FOR GROWING MONOCRYSTALS FROM A MELT IN WHICH THE MELT IS CONTAINED IN A CLOSED VESSEL ROTATABLE WITHIN AN OVEN, THE VESSEL BEING PROVIDED WITH AN SSHAPED DEAERTION TUBE WHICH EXTENDS INTO THE VESSEL THROUGH ITS UPPER END AND HAS AN EFFECTIVE LENGTH FOR REMOVING GASES FROM THE MELT WITHOUT WITHDRAWING MELT WHEN THE VESSEL IS ROTATED.

y 1972 w. TOLKSDORF 3,677,712

DEVICE FOR A METHOD OF GROWING MONQCRYSTALS fig.1

INVENTOR.

WOLFGANG TOLKSDORF BY July 18, 1972 w. TOLKSDORF 3,677,712

DEVICE FOR A METHOD OF GROWING MONOCRYSTALS Filed July 9, 1969 2 Sheets-Sheet 2 INVENTOR.

WOLFGANG TOLKSDORF MA A AGENT United States Patent Office 3,677,712 Patented July 18, 1972 3,677,712 DEVICE FOR A METHOD OF GROWING MONOCRYSTALS Wolfgang Tolksdorf, Tornesch, Germany, assignor to US. Philips Corporation, New York, N.Y. Filed July 9, 1969, Ser. No. 840,331 Int. Cl. B013 17/20 US. Cl. 23-273 2 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a device for and a method of growing monocrystals from an auxiliary melt mass.

In growing monocrystals, high requirements are imposed upon the purity of the starting material, the a-uxiliary melt mass and the material of the crucible so as to obtain monocrystals having optimum magnetic, electric, optical and acoustical properties. For growing yttrium garnet monocrystals it is known to use as an auxiliary melt mass a mixture consisting of lead oxide and lead fluoride. This auxiliary melt mass, however, has the great drawback that the vapour pressure at the dissolving temperature of approximately 1250 C. is high. In this medium the garnet crystals have the tendency of dissolving again below 950 C. It has therefore been tried to separate the crystals outside the oven after a short cooling period, by decanting the excessive liquid at 1040 C. (Electronics, vol. 37, (1964), page 44).

However, this produces a thermal shock. In order to avoid this it is known to decant the auxilitry melt mass at 950 C. in the oven, by making an outflow aperture in the bottom of the crucible (J. Phys. Chem. Solids, vol. 1, (1967),p. 441).

In the known methods the very expensive auxiliary melt mass is either lost entirely or the composition thereof changes by evaporation to such an extent that the melt mass becomes useless. In addition, the melting crucibles which preferably consist of platinum are damaged during operation and/or during opening. This causes loss of material and high costs of conversion.

The invention avoids the problem of a loss of auxiliary melt mass and damage to the melting crucibles. According to the invention this problem is solved by means of a device which is characterized in that a closed vessel is provided in its upper half with a deaeration tube in the form of an S and the vessel is arranged in an oven in a holder so as to be rotatable from without.

As result of the deaeration tube, there is a constant communication between the atmosphere outside the vessel, so that the excessive pressure in the closed vessel, for example, a melting crucible, which pressure is otherwise formed due to the high vapour tension of the auxiliary melt mass is avoided. As a result of this it is avoided that the crucible bulges, springs a leak or cracks, which so far often happened. The S-shaped construction of the deaeration tube has for its result that this tube during operation continuously contains vapour from the auxiliary melt mass. This vapour buffer has a pressurecompensating eflect and avoids an eruptive escape of auxiliary melt mass and vapour from the auxiliary melt mass respectively.

In order to open and close the vessel in a simple manner, for example, by welding, and also to be able to use it again, according to a further embodiment of the invention the closed vessel consists of a hollow cylinder with sealing members, the edge of at least one sealing member extending parallel to the edge of the vessel, the two edges being secured together in an airtight manner.

The method of growing monocrystals from an auxiliary melt mass by means of a device according to the invention is characterized in that the formed crystals are separated from the still liquid remaining melt mass by rotating the vessel in the oven.

In this manner the crystals can be separated from said remaining melt mass after completion of the crystallisation without thermal shocks and without loss of the remaining melt mass.

By means of a device according to the invention it is possible in addition to introduce a seed crystal at a high temperature in the temperature interval of the supersaturation or as short as possible after the spontaneous nucleus formation. For that purpose, a seed crystal arranged inside the vessel above the melt mass is provided in the melt mass by rotating the vessel during the temperature interval of the supersaturation or right after the spontaneous nucleus formation.

In this case the vessel, for example, the melting crucible, must hence be rotated twice totally. Providing the seed crystal during the above-mentioned temperature interval or at the above-mentioned instants is enabled by the rotatability of the melting crucible. This provides the advantage that a solution of the seed crystal in the auxiliary melt mass is avoided, which solution occurs when the seed crystal is situated in the auxiliary mass from the beginning on.

The applicability of the invention is not restricted to the growing of yttrium garnet monocrystals; according to the invention monocrystals of a wide variety, particularly non-congruent melting monocrystals, can be grown from an auxiliary melt mass.

One example of the invention is shown in the drawing, and will be described in detail below with reference to two experiments.

FIG. 1 is a cross-sectional view through the hollow cylinder with the sealing member 2 and FIG. 2 is a diagram of the oven in which the closed vessel and the melting crucible 3, respectively, are arranged in a holder 4.

Experiment 1 A hollow platinum cylinder 1 was closed on one side by means of a closely fitting sealing member 2 having a high edge (FIG. 1) in such manner that the ends of the two platinum parts could be closed by means of an electric welding apparatus. By premelting several times, the filling of 1400 gms. stated in the following table was obtained.

A loss of weight of 24 gms. was obtained which was compensated for by a completion of 16 gms. PbF2 and 8 grns. PbO. With a second similar sealing member 9, in which an. S-shaped deaeration tube (FIG. 2) was welded, the crucible was closed and sealed by welding and provided in a supporting crucible 6 consisting of sinteredaluminum oxide. The intermediate spaces were filled with aluminum silicate wadding 7. This insulation serves for compensating for any small short-lasting temperature variations. FIG. 2 shows the device arranged in the oven.

.The position of the crucible 3 at the beginning of the temperature program is shown in FIG. 2. The oven was heated at 1300 C. (temperature on the bottom of the platinum melting crucible) and cooled according to a temperature program. At 1010" C. the crucible in the furnace was rotated by rotating the suspension shaft projecting outside the oven, which shaft is a component of the holder 4. The crystals grown on the bottom 2 were separated in this manner from the melt mass 8. After cutting open the Welding scam, the bottom which was now at the upper side could be removed. In addition to a few magneto-plumbite crystals (PbFe O four garnet crystals hadgrown. The largest crystal had a weight of 49 grns., the overall yield was 70 gms. which corresponded to 20% of the consumed Y O During crystallisation the weight of the crucible had decreased by gms. Since a little melt mass had come out of the deaeration tube 3, the evaporation loss hence was less than 1% of the consumed lead compounds.

Experiment 2 For Experiment 2, the melt mass 8 and the crucible 3 of Experiment 1 were used. For that purpose the removed bottom 2 was again to the crucible which was again heated in the rotated position, after which the cover 9 with the deaeration tube was taken out, after cutting open of the welding seam. A little melt mass came out of the deaeration tube. The yield of 70 gms, of

a s 'lz of Experiment 1 was replaced and 10 gms. of PbFe were added to compensate for the evaporation losses.

In order to prevent the strong subambient cooling of the melt mass and hence the dendritic growth, a seed crystal was used in this case. For that purpose, a crystal of Experiment 1 was perforated and welded to the cover 9 by means of a platinum wire. The crucible was then sealed by welding by means of said cover and introduced into the oven as described in Experiment 1. The oven was then heated in such manner that the temperature at the bottom of the crucible was .1280 C. and was then cooled to 1250" C. succeeded by a cooling of 05 C. per hour to 1220 C. and by approximately 0.2 C. per hour to 1180" C. At this temperature the crucible was rotated so that the cover 9 provided with the deaeration tube with the seed crystal was located on the lower side. Cooling to 950 C. was carried out at a rate of 0.4" C. per hour, and the crucible was then rotated so that the cover 9 was again uppermost and free from the melting mass. After a complete cooling and opening of the oven, it was found that again four crystals with an overall weight of 52 gms. had grown.

I claim:

1. A device for growing monocrystals from an auxiliary melt mass, comprising an oven, a closed vessel within said oven for containing the melt and means to rotate said vessel within said oven from outside the oven, an S-shaped deaeration tube extending into the upper portion of said vessel and rotatable therewith and having an elfective length substantially coextensive with the vessel for removing gases from the melt without withdrawing the melt when the vessel is rotated.

2. A device as claimed in claim 1 wherein the closed vessel comprises a hollow cylinder, members sealing both ends of the cylinder, the edge of at least one member extending to the edge of the hollow cylinder and the two edges being secured together.

References Cited UNITED STATES PATENTS 503,556 8/1893 Solvay 23-284 3,050,407 8/1962 Nielsen 23-30l 155,948 10/ 1874 Holden 23--286 3,222,142 12/ 1965 Shell et al. 23302 NORMAN YUDKOFF, Primary Examiner S. SILVERBERG, Assistant Examiner US. Cl. X.-R. 23-51, 305 

